Microseismic monitoring is sometimes used to estimate the size and orientation of geologic fractures induced during a hydraulic fracturing operation. Microseismic activity can be measured by placing an array of geophones on the surface surrounding the wellbore or underground in a nearby wellbore. By measuring and mapping the location of small seismic events associated with the growing fracture, the approximate geometry of the fracture can be estimated.
Microseismic analysis is used to provide useful information during the hydraulic fracturing operation and during the subsequent recovery period. The microseismic analysis can be used to quantify the growth of fracture-related stimulation in a reservoir by accurately determining the event hypocenter location. The microseismic analysis can also be used to identify the orientation of the maximum shear stress by calculating stress tensor through the determination of focal mechanisms projected on to the surface array deployment. In some cases, microseismic analysis can be used to estimate the volume expansion through the calculation of the moment tensor and to better understand the fault structure in region.
To accomplish these objectives from a surface-recorded method, it is necessary to have a thorough coverage of the focal sphere with the deployed sensors. In the past, a variety of geometrical arrays have been deployed, most prominently radial arrays (with arms radiating from a central location) as depicted in the drawing in FIG. 1, and patch arrays (with multiple sensors per patch to increase signal-to-noise ratio) as depicted in the drawing of FIG. 2.
Despite these advancements in array configurations, current surface-based microseismic mapping may lack the level of measurement resolution that is required by the technical demands of horizontal drilling and other modern hydrocarbon recovery efforts. The radial array suffers from the limiting azimuthal coverage and from the decrease in sensors further from the well. The patch array also fails to give complete coverage of the focal sphere and requires a large number of patches for its success. Accordingly, there is a need for an improved microseismic mapping system that overcomes the deficiencies of the prior art surface acquisition layout.